This invention relates to improvements in a puffer type gas circuit breaker which, prior to circuit breaking, compresses an arc-suppressing gas and blows the resulting high-pressure gas against the arc produced between the electrodes to extinguish the same.
The puffer type gas circuit breaker is constructed so that, before the contact opening of electrodes that produce an arc during the breaker motion for circuit breaking, a puffer cylinder and a puffer piston fitted therein are moved relative to each other thereby to compress an arc-suppressing gas, e.g., SF.sub.6 gas, in the puffer cylinder, and a blast of highly compressed gas thus obtained is used to blow out the arc that results from the opening of electrode contacts.
It is customary to employ an insulating nozzle to conduct such a highly compressed arc-suppressing gas effectively to the arc occurring between the electrodes, the bore at the throat (the narrowest orifice portion) of the nozzle being made as small as possible to ensure efficient blow-out.
Accordingly, the arcing electrode that passes through the throat (e.g., a fixed electrode where the insulating nozzle is made movable) is small in diameter.
A common disadvantage of the puffer type gas circuit breakers with such slender electrodes is that the breaker motion for circuit breaking tends to invite concentration of an electric field at the free end of the particular electrode.
Especially where a small capacitive current is to be interrupted, it is sometimes done almost simultaneously with the contact opening because of the small current value and low voltage build-up rate. When the circuit is broken in this way, the first peak value of the voltage that varies within the range .sqroot.2E (1 - cos .omega.t) (where .omega. is the angular frequency of power supply, t is the time, and E is the effective line voltage value) is applied in the region of a very short distance between electrodes on the way to circuit breaking, only 0.5 cycle (on the commercial frequency basis) after the contact opening. As a result, restriking of arc can take place due to inability of withstanding the extremely high voltage with the first peak value of 2.sqroot.2E. Such a most objectionable phenomenon tends to occur when the concentration of an electric field at the front end of the slender arcing electrode is serious, particularly at the point of time where the distance between electrodes is very short.
In order to relieve the field concentration at the end of the arcing electrode and prevent restriking of arc, a puffer type gas circuit breaker already known in the art (from Japanese Patent Laid-open No. 25867/1973) uses a movable shield member which surrounds both the first arcing electrode and insulating nozzle and is either connected to a part of the same potential as the first arcing electrode or annularly arranged at substantially the same potential with the said electrode.
The circuit breaker of the known type is constructed to make the circuit by pressing a part of the insulating nozzle against the movable shield member and against the action of a push spring which biases the movable shield member in the circuit breaking direction. Consequently, the movable shield member strikes against the insulating nozzle, and therefore the mechanical strength of the insulating nozzle is a problem. Another disadvantage is an inadequate dielectric strength due to the fact that the movable shield member and movable main contact are connected by the periphery of the insulating nozzle.